1. Field of the Invention
The present invention relates generally to the treatment of subterranean formations penetrated by well bores using aqueous gel fluids. In one embodiment, the invention relates specifically to the use of high viscosity aqueous gel fluids to fracture subterranean formations in order to stimulate the production of hydrocarbons therefrom.
2. Description of the Prior Art
Aqueous gel fluids are used to treat subterranean formations penetrated by well bores in a variety of ways. For example, such fluids are commonly used to temporarily plug or block a portion of a formation to allow a gravel pack to be placed or some other operation to be carried out in the formation. Aqueous gel fluids are also commonly used to fracture subterranean formations in order to stimulate the production of fluids, such as petroleum, therefrom.
In a fracturing treatment, an aqueous gel fluid is injected down the well bore at a flow rate and pressure sufficient to develop hydraulic forces which form a fracture in the formation. The gel quality of the fluid aids in the suspension of proppant material, e.g., sand, in the fluid and the formation of the fracture. Elevated pressure is maintained on the fluid for a sufficient time to cause movement of the fluid into the fracture and thereby propagate or extend the fracture into the formation. The proppant material in the fluid holds the fracture open upon release of the pressure. Crosslinking agents are typically used to increase the viscosity of the fluid which facilitates transport and placement of the proppant material and the formation of an extended fracture.
Regardless of the type of treatment, it is generally desirable to recover the aqueous gel fluid from the formation once the treatment is complete and prior to initiating or resuming production. In order to recover the fluid, it is often necessary to first reduce the viscosity of the fluid by causing or allowing the gelling agents in the fluid to break down. This process is commonly referred to as breaking the gel or breaking the aqueous gel fluid. The process can be accomplished by a number of methods. For example, breakers consisting of mild oxidizing agents or enzymes can be included in the aqueous gel fluid to break the fluid at a certain time. The breaking time can generally be predicted within relatively narrow limits. In high temperature applications, the aqueous gel fluid will often break on its own over time. Once the fluid is broken, it is recovered from the formation by way of the well bore.
Unfortunately, the process of breaking the aqueous gel fluid creates very small (e.g., microscopic) broken gel fragments (gel residue) which tend to agglomerate or flocculate and form relatively large broken gel masses. These masses can severely damage the formation. For example, the masses can plug proppant packs formed in fractures severely reducing the fracture flow capacity of the formation. Also, oil present when the gel fluid is broken can be incorporated into the masses leaving the broken gel fragments oil-wet. The oil-wet nature of the fragments aggravates the problem and gives the broken gel fluid an extremely dirty, damaging appearance.
There is a need for an aqueous gel fluid and method of treating a subterranean formation with the same in which broken gel fragments formed when the fluid breaks do not decrease the effectiveness of the treatment.